The vireo is 11–14 cm (4–6 inches) in length, with a gray head, back, and flanks, and whitish underparts. It has solid white "spectacles" and white wing bars.

The song, given persistently, consists of short, rough whistled phrases of several notes, spaced about 2 seconds apart. The phrases often alternate ending on a high note and a low note, giving an impression of question and answer.

It prefers open woodlands of the western mountains and foothills. It is usually found in the middle to lower portions of the forest canopy, where it slowly and deliberately forages for insects among the foliage.

Cassin's vireo builds a cup nest out of bark strips and down in the fork of a twig. It lays 2 to 5 white eggs with some brown spots.